Sighting telescopes are used in hunting and for military purposes to aim weapons at distant targets. For that purpose they are fitted with a set of lens elements configured within a housing and magnifying a target. As a result, the field of view is substantially limited and aiming at relatively nearby targets is degraded. In order that such nearby targets also may be aimed at, the state of the art offers variable magnification, i.e. zoom lenses. Also the sighted target is laterally inverted as well as upside down and therefore requires corresponding correction.
A reversing system configured within the sighting telescope is used to implement such a correction. It allows axially displacing at least one lens element within the reversing system, and as a rule further allows independent/defined displacement of two optical lens elements. Such lens elements include reticles and elements cemented to each other. This a design erects the image which is reproduced in the observed ocular plane. Present days sighting telescopes configure the reticle in the plane of the overall lens (objective) or in the ocular plane, the reversing system being configured within an inner tube of the sighting telescope.
Such a reversing system in general is made up of a guide casing fitted with a linear cam slot and of an external casing fitted with a first external cam slot and optionally with a second external cam slot, the external casing resting rotatably on the inner guide casing while being axially fixed in position. Said linear cam slot is constituted by a single straight slot running parallel to the guide casing's center axis. On the other hand the two external cam slots subtend a curved path. Moreover a first and a second mount holding two lens elements are supported in displaceable manner in said guide casing. A first guide pin is connected to the first mount and is configured at least partly within the linear cam slot and the first external cam slot. In equivalent manner, a second guide pin is linked to the second mount and configured at east in part within the linear cam slot and the second external cam slot. As a result the spacings between the lens elements of the optical reversing system and the objective plane of the sighting telescope are variable. As a result, optical magnification may be adjusted and the image no longer is upside down.
It is critical that the lens elements be configured in their design positions very accurately in order to create a sharp image for the marksman. Already positional deviations as small as 0.01 mm degrade image quality and entail parallax errors. In other words, the target mark and the actual projectile impact point deviate from each other. Accordingly all movable parts must be designed with very narrow tolerances and there must not be, so to speak, any play between them. Excess dimensions frequently are resorted to in order to preclude play. As a result the reversing system presents difficulties in adjusting it. The contact zone between the guide pins and cam slots is especially critical.
Illustratively metal screws are used in the state of the art to engage the guide pins into the cam slots. The external casing fitted with the curved cam slots also is metallic. A certain intrinsic play is present in such a combination of metal screws and metallic external casing, and as a result of this play, the metal screw may be moved within the curved cam slot. However such an intrinsic play is disadvantageous in that the reversing lens elements are not arrayed in a defined manner and therefore may cause parallax deviation. If said intrinsic play is minimized, weather-determined thermal fluctuations entail jamming on account of the parts' thermal expansion. This condition not only prevents instantaneous adjustability, but also causes permanent damage when forcefully acted on by the marksman. Such problems are encountered at very low temperatures, for instance as low as −40° C. (−40° F.).
In a further design of the state of the art, the guide pin and the external casing both are metallic, however the guide pin is partly fitted with a plastic cladding. The diameter of the guide pin in said cladded zone evinces a diameter larger than the width of the cam slots. By means of its oversized cladding, the guide pin enters the linear cam slot and the external cam slot. As a result, in the first place, the play causing the parallaxes is precluded. On the other hand it has been observed that such a reversing system also will jam at very low temperatures.
Furthermore the German patent document DE 20 2006 000 977 U1 discloses a guide pin, in particular a guide screw, comprising a first cladding element and a second cladding element to enclose the screw. The first cladding element is configured on the screw and the second cladding element is configured on the first cladding element. Also the second cladding element is oversized relative to the cam slot, but not the first one. Accordingly installing the second cladding element will warp it. In further plastic deformation of the second cladding element caused by firing the weapon the warping caused by the first cladding element is limited. For that purpose the first cladding element is fitted with a shoulder which, following exceeding a given plastic deformation of the second cladding element, will brace the guide pin against at least one of the cam slots. Consequently the first cladding element will touch the second one only when latter is highly stressed.
This design however also incurs a drawback, namely that a reversing system fitted with such a guide peg shall jam at very low temperatures, in particular because then the plastic becomes brittle and inelastic. As a result, due to oversizing and the different thetinal coefficients of expansion of the materials used and hence of the parts, the second cladding element may break.